1. Field of the Invention
The present invention relates to a shape measuring apparatus having a stylus type probe capable of measuring a surface shape of an optical element such as a lens and a mirror, and a mold for manufacturing the optical device at a high precision on the order of nanometer, and a shape measuring method thereof. More particularly, the present invention relates to a shape measuring apparatus capable of coping with measurement of a shape having a steep inclination angle, for example, a shear wall surface rising vertical from horizontal.
2. Description of the Related Art
Generally, as a shape measuring method for measuring coordinates or a shape of a specific portion of the surface of a measured object having a 3-dimensional shape, a measuring method using the stylus referred to as probe has been known. According to this measuring method, the probe is allowed to trace the surface of the measured object while pressed against the surface of the measured object at a predetermined contact force, so that, by measuring a position of the probe with respect to a predetermined origin and an orientation of the probe, the shape of the measured object is measured.
Conventionally, as regards such a shape measuring method, a contact type probe based on a shape measuring method as discussed in Japanese Patent Application Laid-Open No. 2005-37197 has been known to those skilled in the art. According to this method, a stylus probe is used to measure the shape of a measured object, the probe being supported with a leaf spring suspended from a housing. The probe is provided with a displacement sensor capable of measuring a relative position of the probe with respect to the housing. Based on these sensors and preliminarily measured stiffness (spring constant) in each direction of the suspended leaf spring, a contact force can be measured based on a displacement generated when the probe comes into contact with the measured object. By synthesizing obtained contact forces Fx, Fy, Fz of respective directions, a normal force acting against the probe from the measured object can be estimated. The probe is allowed to trace a profile of the measured object with the magnitude of a contact force F (normal force) of the probe kept constant to measure the shape. Thus, according to the shape measuring method discussed in Japanese Patent Application Laid-Open No. 2005-37197, even if the measured object has a steeply-inclined surface, its shape can be measured with the contact force applied from the probe to the measured object kept constant. Accordingly, the shape can be measured while generation of a system error accompanying an increase and decrease in the contact force during the measurement depending on the inclination of the measured object surface is reduced.
However, according to the method discussed in Japanese Patent Application Laid-Open No. 2005-37197, the housing is moved in the horizontal direction along a fixed scanning track to scan the probe, and the housing driven by a servo motor is displaced in the vertical direction. Consequently, a contact force F (normal force) applied to the probe can be controlled. Thus, when the measured object surface has a steep slope, for example, a vertical surface, namely when the displacement direction of the housing for controlling the force is substantially parallel to the tangent line of the slope of the measured object, the contact force to the measured object changes little even if the housing is displaced in the vertical direction, thereby making it difficult to control the contact force.
When the probe is moved in the horizontal direction along the scanning track and the probe comes into contact with the vertical surface of the measured object, of a force S urged against the probe by a movement of the housing, a component force St in the direction of the tangent to the measured object is so small that the probe cannot move, thereby making it difficult to scan the probe along the profile of the measured object.
Therefore, when the probe is kept in contact with a steep surface, for example, a vertical surface while controlling the contact force F applied to the probe by displacing the housing in the vertical direction, it is difficult to allow the probe tracing the profile of the measured object, thereby making it difficult to measure the shape.